Protein phosphorylation is a key event in almost all signaling pathways, and mediates much of the information transfer and processing that occur in these pathways. Misregulation of phosphorylation has been implicated in a number of diseases, particularly in cancer. However, existing tools for measuring protein phosphorylation in vivo are unable to monitor phosphorylation of many proteins simultaneously and in real time. We aim to build a system to measure the phosphorylation of hundreds of proteins, in vivo, in real time. To do so, we are building a system that can detect phosphorylation of a YFP tagged protein by fluorescence resonance energy transfer to a CFP-tagged phosphopeptide binding domain. We will use this system to study phosphorylation events in the yeast mating pathway, a prototype eukaryotic signaling pathway. Real-time measurements of phosphorylation in this system will enable us to monitor the quantitative response of this pathway and its activation of downstream effectors.